Originally Posted by Bob Krispen
<firstname.lastname@example.org>: [mail.example.com]: Name or service not known
I've confirmed that 'mail.example.com' resolves on the server and it correctly identifies the external IP address of this server as that mail server.
This means that your DNS records aren't set up correctly, or that they need some more time to propagate (if you've changed the DNS records recently, it might take up to 3 days to propagate).
Can you tell me your real domain and IP address so that I can check?